1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor thin formed on a substrate having an insulating surface and a semiconductor device using the semiconductor thin film as an active layer. Particularly, the present invention relates to a semiconductor thin film in which an amorphous semiconductor thin film containing silicon as its main ingredient is crystallized to obtain a thin film.
Moreover, the present invention relates to the structure of a semiconductor circuit and an electro-optical device, which are constituted by a semiconductor device such as a thin film transistor, and to the structure of an electronic apparatus incorporating those.
Incidentally, in the present specification, all of the thin film transistor, semiconductor circuit, electro-optical device, and electronic apparatus are contained in the category of xe2x80x9csemiconductor devicexe2x80x9d. That is, any device capable of functioning by using semiconductor properties is called a semiconductor device. Thus, the term xe2x80x9csemiconductor devicexe2x80x9d included in the present invention includes not only a single element such as a thin film transistor but also a semiconductor circuit obtained by integrating the single elements, an electro-optical device, and an electronic apparatus incorporating those as parts.
2. Description of the Related Art
In recent years, attention has been paid to a technique for constituting a thin film transistor (TFT) by using a semiconductor thin film (its thickness is several tens to several hundreds nm) formed on a substrate having an insulating surface. Particularly, development of the thin film transistor as a switching element of an image display device (for example, a liquid crystal display device) has been hastened.
For example, in a liquid crystal display device, trials have been made to apply TFTs to any electric circuit, such as a pixel matrix circuit for controlling each of pixel regions arranged in matrix, a driving circuit for controlling the pixel matrix circuit, and a logic circuit (arithmetic circuit, memory circuit, clock generator, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using a noncrystalline silicon film (amorphous silicon film ) as an active layer is put to practical use, a TFT using a crystalline silicon film (polysilicon film, etc.) is necessary for an electric circuit, such as a driving circuit or a logic circuit, required to have higher speed operation performance.
Conventionally, a crystalline silicon film is obtained in such a manner that heat treatment, irradiation of laser light, or irradiation of intense light is carried out after an amorphous silicon film is formed on a substrate having an insulating surface or an under film having an insulating surface by a plasma CVD method or a low pressure CVD method.
Among the above conventional methods of obtaining a crystalline silicon film, the quality of a film obtained by the method of irradiation of laser light is excellent as compared with other methods, and the method has a high throughput and has a merit that thermal damage is not caused to a substrate, so that the method is often used.
However, according to the method of irradiation of laser light, if the thickness of an amorphous silicon film is 100 nm or less, many ridges (asperities) are formed on the surface of the obtained crystalline silicon film so that the film quality is degraded. That is, when a silicon film is irradiated with laser light, the silicon film is instantaneously melted and is locally expanded, and ridges (asperities) are formed on the surface of the obtained crystalline silicon film to relieve the inner stress generated by this expansion. The difference in the height of the ridge is about xc2xd to 1 time the thickness of the film.
In an insulated gate semiconductor device, since a potential barrier or a trap level caused by a dangling bond, distortion of a lattice, or the like are formed in the ridge on the surface of the crystalline silicon film, an interfacial level between an active layer and a gate insulating film is raised. Further, since the top portion of the ridge is steep, an electric field is apt to be concentrated, so that the ridge becomes a generating source of leak current, and finally, breakdown occurs and a short circuit is brought about. In addition, the ridge on the surface of the crystalline silicon film damages the covering properties of the gate insulating film deposited by a sputtering method or a CVD method, and causes poor insulation or the like to degrade the reliability. Thus, the ridge on the surface of the crystalline silicon film influences all the characteristics of a TFT and even a yield is changed.
Moreover, the method of irradiation of laser light is apt to become unstable particularly under a condition to obtain excellent crystallinity, and if the energy density of laser light is increased to make sufficient crystallization, there is a tendency that ridges are increased and the surface of a film becomes rough.
An object of the present invention is to solve the above problems and to provide a semiconductor device which has high characteristics and uses a crystalline silicon film having high crystallinity and having a flat surface with few ridges (asperities), and a method of manufacturing the same.
According to a first aspect of the present invention, a method of manufacturing a semiconductor device is characterized by comprising the steps of: forming a first amorphous silicon film on an insulating surface; obtaining a first crystalline silicon film by carrying out a heat treatment to crystallize the first amorphous silicon film; forming a second amorphous silicon film on the first crystalline silicon film; and obtaining a second crystalline silicon film by applying an energy to crystallize the second amorphous silicon film.
According to a second aspect of the present invention, a method of manufacturing a semiconductor device is characterized by comprising the steps of: forming a first amorphous silicon film on an insulating surface; obtaining a first crystalline silicon film by carrying out a heat treatment to crystallize the first amorphous silicon film; etching a surface of the first crystalline silicon film; forming a second amorphous silicon film on the first crystalline silicon film; and obtaining a second crystalline silicon film by applying an energy to crystallize the second amorphous silicon film.
In the second aspect of the present invention, an etchant containing hydrofluoric acid is used as an etchant in the etching step.
According to a third aspect of the present invention, a method of manufacturing a semiconductor device is characterized by comprising the steps of: forming a first amorphous silicon film on an insulating surface; introducing a metal element for facilitating crystallization of silicon into the first amorphous silicon film; obtaining a first crystalline silicon film by carrying out a heat treatment to crystallize the first amorphous silicon film; forming a second amorphous silicon film on the first crystalline silicon film; and obtaining a second crystalline silicon film by applying an energy to crystallize the second amorphous silicon film.
In the third aspect of the present invention, it is characterized in that one kind of or plural kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, and Au are used as a metal element for facilitating crystallization of silicon.
Moreover, in the third aspect of the present invention, it is preferable to use nickel as the metal element for facilitating crystallization of silicon.
Moreover, in each of the above aspects of the present invention, crystallization of the second amorphous silicon film is carried out by using the surface of the first crystalline silicon film as nuclei of crystal growth.
Moreover, in each of the above aspects, it is characterized in that, as a method of applying the energy, a method of irradiation of laser light is used. Moreover, as a method of applying the energy, in addition to irradiation of laser light, it is preferable to use one kind of or plural kinds of methods selected from irradiation of intense light and heating concurrently or sequentially. Moreover, in each of the above aspects, it is preferable that the irradiation energy density of the laser light is 100 to 300 mJ/cm2.
According to a fourth aspect of the present invention, in a semiconductor device comprising an active layer made of a semiconductor thin film formed on an insulating substrate, a gate insulating film, and a gate electrode, it is characterized in that the active layer includes a laminated structure made of a first crystalline silicon film and a second crystalline silicon film stacked on the first crystalline silicon film; the first crystalline silicon film includes a crystal structure obtained by crystallization through heating; and the second crystalline silicon film has a crystal structure obtained by crystallization through irradiation of laser light.
In the above fourth aspect of the present invention, it is characterized in that the crystal structure obtained by crystallization through heating includes thin rod-like crystals or flattened rod-like crystals.
Moreover, in the fourth aspect of the present invention, it is characterized in that the crystal structure obtained by crystallization through heating is such that thin rod-like crystals or flattened rod-like crystals are grown with intervals and in parallel to or substantially in parallel to each other.
In the present invention, although any well-known means may be used to crystallize the first amorphous silicon film to form the first crystalline silicon film, crystallization through heat treatment is preferable. The thus obtained first crystalline silicon film is used as an under film, the second amorphous silicon film is formed thereon, and the second crystalline silicon film is formed by irradiation of laser light, so that excellent flatness can be obtained. Moreover, at the irradiation of the laser light, an irradiated region may be heated at a temperature ranging from 450xc2x0 C. to the distortion point of the substrate to make a step of obtaining further excellent crystallinity.
The present invention is characterized in that the second crystalline silicon film is obtained by irradiation of laser light while using the surface of the first crystalline silicon film as nuclei of crystal growth. Thus, the second crystalline silicon film is influenced by the surface of the first crystalline silicon film. That is, if the surface of the first crystalline silicon film is excellent as nuclei, the second crystalline silicon film having excellent crystallinity and flatness can be obtained. Accordingly, it is preferable to form an excellent surface by etching the surface of the first crystalline silicon film when the second amorphous silicon film is formed. Alternatively, it is preferable to form the second amorphous silicon film while an excellent surface of the first amorphous silicon film immediately after crystallization is maintained. When the thus obtained two-layer crystalline silicon film is used for an active layer of a thin film transistor, a semiconductor device having superior characteristics can be obtained.
As compared with a conventional crystalline silicon film in which an amorphous silicon film formed on an insulating film of SiO2 or the like is crystallized by irradiation of laser light, in the present invention, it is possible to obtain an excellent crystalline silicon film having a flat surface with fewer ridges.
Although the film qualities of the first amorphous silicon film and the second amorphous silicon film are almost the same, the first crystalline silicon film and the second crystalline silicon film are different from each other in crystal grain boundaries, that is, in the crystal structure, which is one of the features of the present invention.
This can be confirmed by secoetching (as an etchant, a mixture solution of HF=50 cc, K2Cr2O7=1.14 g, and H2O=25 cc). When this secoetching is carried out, it is possible to observe defects and crystal grain boundaries on the surface by SEM observation or the like.
That is, the surface structure of the first crystalline silicon film crystallized by a heat treatment has an irregular crystal structure as shown in FIG. 14A in which an example is shown, and the respective crystals do not have regularity. FIG. 14A is a SEM observation photograph of a surface of a crystalline silicon film which was obtained by only a heat treatment (600xc2x0 C., 24 hours) and was subjected to secoetching.
Moreover, in the case where the first crystalline silicon film is crystallized by addition of a catalytic element and by a heat treatment (using the technical content of Embodiment 1 of Japanese Patent Laid-open No. Hei. 7-130652 having U.S. Pat. No. 5,643,826 corresponding thereto, which are herein incorporated by reference), although not shown, crystals grow radially from a limitless number of point centers in the whole surface of the film, and the respective radial crystals grow like rods in which crystal lattices are continuous.
Moreover, in the case where the first crystalline silicon film is crystallized by addition of a catalytic element and by a heat treatment (using the technical content of Embodiment 2 of Japanese Patent Laid-open No. Hei. 7-130652), although not shown, the structure of crystal lattices is continuous in almost a specific direction, and thin rod-like crystals or thin flattened rod-like crystals are grown.
As compared with these foregoing first crystalline silicon films, the crystal structure of the surface of the second crystalline silicon film is quite different. FIG. 14B is a SEM observation photograph of a surface of a conventional crystalline silicon film which was obtained by irradiation of laser light (340 mJ/cm2) and was subjected to secoetching. As shown in FIG. 14B in which one example of the patterns is shown, the film has regular (like tortoiseshell pattern) crystal grain boundaries.
The second crystalline silicon film of the present invention is characterized in that it has fewer ridges than the prior art (FIG. 14B) and has excellent flatness.
In the present specification, the crystal structure obtained by crystallization through heating indicates a structure one example of which is shown in the pattern of FIG. 14A. That is, each of the crystals in the crystal structure is constituted by irregular crystal grains, thin rod-like crystal grains, or thin flattened rod-like crystal grains.
Moreover, in the present specification, the crystal structure obtained by crystallization through irradiation of laser light indicates a structure one example of which is shown in the pattern of FIG. 14B. That is, the crystal structure is constituted by regular (like tortoiseshell pattern) crystal grains.